The use of wet, multi-plate brake systems for vehicle braking has been historically known. Early systems employed a mechanical arrangement of levers, links and/or cams responsive to brake pedal pressure to compress the brake pack and decelerate the output member.
Later braking system developments combined a hydraulic apply system with a mechanical apply system to reduce required pedal effort for a given deceleration rate. The forces generated by the hydraulic and the mechanical apply systems combined to compress the multi-plate brake packs employed with individual transmission output shafts, or the axle assemblies connected thereto. Such brake packs are generally actuated by axial compression to effect the desired braking action in response to depression of the brake pedal. That is, the compression of the multiple, interleaved torque plates, with the associated friction disks therebetween, effects the torque transfer which actually slows the vehicle.
More specifically, a system of cams and levers between the apply shaft and a brake apply member causes the latter member to rotate in proportion to rotation of the apply shaft. Rollers are received between two ramps--one of which is on the brake apply member that is adjacent to, and engages, the brake pack and the other of which is attached to ground. The opposed ramps engage the rollers and are circumferentially inclined such that when the brakes are applied, the brake apply member is forced against the brake pack. This results in a mechanical compression of the brake pack, and the mechanical force at the brake is proportional to the force on the brake pedal.
The mechanical operation results in significant self-energization by virtue of the frictional interaction between the torque plates within the brake pack. Self-energization can, therefore, cause the brake pack to effect a deceleration rate in excess of that desired, or expected. This result is compounded by the hydraulic portion of such a combined system. Combined systems include an arrangement wherein one of the apply shafts also actuates a hydraulic valve which, in turn, delivers brake apply pressure to pistons associated with the brake apply member. The hydraulically generated pressure acting against the brake apply member also forces the brake apply member against the brake pack, thereby adding an additional force to the force generated by the mechanical aspect of the combined braking system. The pressure generated by such a system does not provide a feed back force at the pedal that is in any way proportional to, or reflective of, the hydraulically generated brake apply pressure.
The customary timing of a combined mechanical and hydraulic system is such that the initial rotation of the apply shaft results in mechanically stroking the brake apply member. The hydraulic apply is normally delayed until after the apply shaft has been rotated sufficiently to signal the brake coolant valve, thus assuring that cooling fluid--normally the lubricating oil--is present before the clearance between the torque plates, and the associated friction disks, in the brake packs is completely removed. During this first few degrees of relative rotation between the friction disks in the brake pack, the brake return springs are compressed by the mechanical apply system. This compression of the components in the mechanical apply system results in a linear force feed-back to the driver's foot.
However, when the hydraulic apply begins to provide brake pressure, it assumes the load from the mechanical system but does not provide a concomitant feed-back to the driver's pedal. Thereafter, the apply pressure is purely hydraulic until the hydraulic valve is fully stroked and the brake pressure is at its maximum. Once the maximum hydraulic application is achieved, operation of the mechanical aspect of the combined system returns. That is, rotation of the brake apply member brings the rollers into contact with the opposed ramps such that the application of additional torque on the apply shaft results in a proportionately increased mechanical force of the brake apply member.
The problem is that the tendency of the driver is to "push through" the initial point where the resistance to his foot is greatly reduced. This results in an unintended increase in the deceleration rate as the additional, unintentional stroking of the hydraulic apply system increases the apply pressure. The result can be unexpected, sudden braking.
Various solutions have been sought for this problem, such as, for example, applying the service brake solely by virtue of the hydraulic system and using a separate apply shaft to actuate the mechanical system solely as a parking and emergency brake that must be applied by a hand-actuated lever. One difficulty with this approach, of course, is that, in the event of hydraulic failure, the driver must react quickly to find and apply the emergency brake in order to bring the vehicle to a stop.
Another possible problem with this approach is that the hydraulic valve will not fill and stroke the brake piston at a sufficiently rapid rate for partial brake application. Furthermore, during a full apply, the valves of the hydraulic system are fully stroked, resulting in a rapid apply time for the brakes. However, during a partial apply, as in a request for a slow rate of deceleration, these valves are only partially stroked. This creates a flow restriction past one of the valves, thus increasing the delay time of initial brake pack closing and, therefore, may delay brake response to unacceptable limits.
It is, therefore, deemed desirable to provide a system which takes full advantage of the self-energization factor of the mechanical application system, but permits the application of an opposing hydraulic pressure that is regulated by an electronic system which senses the desired deceleration rate in response to the degree to which the brake pedal is depressed.